Energy efficiency is a major concern that affects nearly every aspect of society. Industrial machinery and transportation in particular are heavy consumers of portable energy through the use of electricity, gasoline, diesel or natural gas powered motors. Most energy from a transportation energy source dissipates as heat because of inefficiencies during chemical energy conversion into mechanical work. A major inefficiency is the mismatch between a faster rotating motor shaft or gear and a slower rotating device that receives such energy such as a wheel of a car or propeller of a boat.
A variety of transmission systems have been developed to minimize these losses. Unfortunately, each system has its own inefficiencies and problems. For example, in the case of powered watercraft that employ a fixed gear ratio, energy is lost from friction in the reducing gear and also in the propeller of such drive systems because the small propellers used represent a compromise and rotate at a much higher than ideal rate to push water efficiently. Ideally, a fast rotating motor with a high power output and with shaft speed of about 3,000 or 4,000 rpm should be geared down to a much slower rpm of a few hundred rpm, but with higher torque as needed to push water with a (preferably) large, slowly revolving propeller. Inexpensive gears and transmissions generally are not available for such high ratio speed changes. Accordingly, modern pleasure watercraft at low to medium speed generally are operated at lower than desired efficiencies.
David Geer has described this low efficiency problem of moderate speed watercraft (Propeller Handbook page 79) as “[f]or a given horsepower, the slower the shaft RPM and the larger the diameter the more efficient the propeller will be. This is true for every installation, unless the boat speed will consistently be above 30 or 35 knots. Accordingly, in selecting a propeller you should always start with the largest diameter possible for the given hull, and work from there. . . . Draft limitations, hull shape, and tip clearances . . . are nearly the only factors that should cause you to consider a smaller diameter for slow-to-moderate speed craft. Another practical limitation is that while reduction gears with ratios as great as 6 or 7 to 1 are available for larger marine engines of, say over 250 hp (185 kw). standard reduction gears . . . are seldom available with ratios larger than 3 to 1 . . . . ″ According to this reasoning, a highly efficient and simple gear reduction of greater ratios approaching 10 or even 20 fold would give great benefits for many watercraft but is not readily available for regular watercraft.
A related problem is the need to rapidly stop a propeller, conveyor or other equipment upon detection of an unsafe condition. For example, a spinning propeller poses great hazards to swimmers and other waterlife. A rapid propeller stop system, is highly desirable but generally not considered because of the extreme difficulty in rapidly stopping a propeller. A limitation in this regard is that most propeller shafts are permanently fixed to a motor, either directly or indirectly through reduction gearing and rapid stoppage would overstress the drive system, due to the inertia of moving parts. Although not generally appreciated, a power transmission link between motor and propeller that both provides a high rotational speed change and the ability to rapidly stop a connected propeller would potentiate technological advances in electronic propeller guard systems. Unfortunately, such system generally is not available.
Such systems, if available could save lives. According to statistics kept by the U.S. Coast Guard, scores of people are killed or severely maimed each year from propeller injuries. Other mammals such as manatees are severely injured and disfigured and this problem threatens the tourism industry in areas such as Homosassa Springs State Park in Florida. The boating industry has struggled with this problem without much success for some time. The often proposed solution of using a mechanical propeller guard to physically block contact, while logical at first glance actually is very impractical, despite a number of attempts to implement this idea as described in U.S. Pat. Nos. 3,889,624; 4,411,631; 44,826,461; 4,078,516; 5,238,432; 4,957,4459; 5,009,620; 4,304,558; 5,759,075; 4,565,533; and 4,106,425. The guard would rob too much propulsion power and in some cases could increase the occurrence and severity of propeller injuries because the guard can act as a catch that prevents easy removal of a hand or foot from the propeller vicinity as commented on, for example by the Superior Court of Pennsylvania (Fitzpatric v. Madonna, 623 Aa.2d 322 1993), which stated that “the presence of a shroud over the propeller presents its own risks for swimmers. For example, a shroud creates a larger target area. In addition, the possibility exists that human limbs may become wedged between a shroud and the propeller, exposing a swimmer to even greater injury.” Accordingly, a safer system is desired that can rapidly stop a propeller.
A large variety of gear reducers, clutches and other power transmission devices have been developed for many transportation machines. New types of clutches have evolved particularly for fans and air conditioners on cars and trucks and have provided incremental but highly desirable efficiency improvements for some applications. For example, a series of patents from Larry Link describe an electric clutch that electromagnetically disengages a fan as needed to minimize drag on an engine when the cooling fan is not required. See, for example, U.S. Pat. Nos. 6,129,193; 6,230,866; 6,331,743 and 5,947,248; which teach the use of radially disposed electromagnets and a concentric set of pole pieces separated by an air gap. The torque transfer is modulated by controlling electric power to the multiple radially disposed electromagnets. This system promises to overcome frictional losses engendered by the widely used viscous clutch systems. However, the Link device appears to generate a considerable amount of heat, the electromagnets generally are rotating and need an electrical supply through a slip ring, and the entire system requires numerous parts. Furthermore, the energy efficiency of the Link system, which is notable by its omission from the copious documents that describe this technology, apparently is low. This view is supported by the Link disclosures, which emphasize multiple features that generally had to be added to remove heat buildup from the frictional losses, which again indicate that the system is inefficient.
Magnetic systems have been described for coupling other rotating axles as well. Masberg et al. (U.S. Pat. No. 6,149,544) teaches a coaxial (rotating cylinder within a rotating cylinder) dual electromagnet system that offers a stator body and a housing, which in some embodiments resembles a motor that couples two axles as a magnetically controlled clutch. This system is complex and generally requires a three dimensional magnetic assembly that maintains close tolerances in a dimension along the axis of rotation. Magnetic fields interact that are perpendicular to the rotational axis. The device is not unlike that of a regular induction motor, with the armature connected to a first axle and the field coil rotating and connected to a second axle.
Another interesting coaxial electromagnetic coupler is taught by U.S. Pat. No. 5,565,723, which emphasizes an internal electrical feedback to obtain a desired torque speed characteristic. The apparatus taught in this patent also uses two coaxially oriented rotable parts with inner and outer cylinders of electromagnets that exert magnetic coupling forces, which are perpendicular to the axis of rotation. This system as well appears very complex, and has slip rings to apply electricity to moving electromagnets. Such complexity is undesirable, particularly for applications in the marine environment, where exposed electrical connections and conductors need to be marinized.
Despite a wealth of technology in the automotive and related arts, transmissions that provide high gear ratios and inexpensive, durable rapid acting clutches are not widely used for regular pleasure watercraft and other applications such as screw conveyors, elevators and related devices. In the case of watercraft, durable and cost competitive gear reducers of gear ratios less than 4 to 1 generally are used and rapid disconnect of propellers from the drive train is not carried out because of technology and cost limitations. While not recognized as such, these limitations are taken for granted and specific watercraft installations are optimized with inherent built in equipment limitations. For example, a specific boat with a specific boat motor generally is matched with a specific propeller that meets a selected criteria for best torque, motor speed, and motor output for a single optimum boat speed. Consequently, most drive systems are limited to a single gear reduction ratio and a single optimum propeller/boat combination that is chosen partly based on such a specific combination.
Similar limitations exist for other applications such as saws, conveyors and vehicles. Any device that provides greater flexibility in torque conversion between an upstream driving axle, such as a crankshaft or other drive gear and a downstream axle, such as a propeller shaft or other gear would advance the art of mechanical energy conversion by allowing a broader range of conditions for optimization. In the example of a torque converter for a propeller driven watercraft, better optimization of boat speed for optimum efficiency, and motor or motor conditions would be possible if a suitable torque converter were available that was efficient over a wide range.